Gas sensor control apparatus

ABSTRACT

A gas sensor control apparatus ( 1 ) for a gas sensor ( 2 ) including an electromotive force cell ( 24 ) and a pump cell ( 14 ) includes current control means ( 69 ) for feedback-controlling the pump current Ip, voltage setting means S 5 , S 13  for setting a target voltage Vr to either of first and second target voltage Vr 1  and Vr 2 , and constant group setting means S 4 , S 12  for setting a group of feedback control constants to a first group Kpid 1  when the target voltage is Vr 1  and to a second group Kpid 2  when the target voltage is Vr 2 . The second group Kpid 2  is determined such the pump current Ip becomes stable more quickly as compared with the case where the first group of control constants Kpid 1  continues to be used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor control apparatus whichalso detects the H₂O gas concentration of a gas under measurement byusing a gas sensor which detects the concentration of a specific gascontained in the gas under measurement.

2. Description of the Related Art

Examples of conventionally known gas sensors for detecting theconcentration of a specific gas contained in a gas under measurement,such as exhaust gas discharged from an internal combustion engine,include an oxygen sensor for detecting the concentration of oxygen andan NOx sensor for detecting the concentration of nitrogen oxide (NOx).These gas sensors include a sensor element(s) composed of a solidelectrolyte body mainly made of zirconia. For example, a full-rangeair-fuel ratio sensor whose output changes linearly with oxygenconcentration has two sensor elements; i.e., an electromotive force celland a pump cell. The current flowing between the electrodes of the pumpcell is controlled such that the voltage generated between theelectrodes of the electromotive force cell becomes constant, and theoxygen concentration is detected from the magnitude of the currentflowing through the pump cell.

Patent Document 1 discloses a gas concentration/humidity detectionapparatus which detects not only the concentration of a specific gascontained in a gas under measurement, but also the humidity of the gasunder measurement. In this gas sensor, the detected concentration of thespecific gas (e.g., oxygen concentration) is corrected on the basis ofthe detected humidity.

In the gas concentration/humidity detection apparatus disclosed inPatent Document 1, when the humidity of the gas under measurement(namely, the concentration of H₂O gas contained in the gas undermeasurement) is detected, the control target voltage of theelectromotive force cell is switched from a first reference voltage(e.g., 450 mV) at which the H₂O gas contained in the gas undermeasurement does not substantially dissociate, to a second referencevoltage (e.g., 1000 mV) at which the H₂O gas contained in the gas undermeasurement dissociates. The H₂O gas concentration is detected on thebasis of first and second currents which are pump currents detected whenthe first and second reference voltages are used, respectively.

-   [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No    2010-281732

Problems to be Solved by the Invention

However, after the control target voltage is switched from the firstreference voltage to the second reference voltage, it is necessary towait for a relatively long time (e.g., several to about 10 sec) beforethe pump current under feedback control becomes stable such that aproper second current can be obtained. Meanwhile, during a period duringwhich the H₂O gas concentration is detected, the oxygen concentrationcannot be detected. Therefore, during this period feedback control ofair-fuel ratio for an engine using the oxygen concentration output(air-fuel ratio output) of the gas sensor cannot be performed, and theair-fuel ratio is subject to open-loop control. Therefore, there is aneed to shorten, to the extent possible, the time required for properlydetecting the H₂O gas concentration.

SUMMARY OF THE INVENTION

The present invention has been accomplished in order to solve the aboveproblems, and an object thereof is to provide a gas sensor controlapparatus which uses a gas sensor so as to detect the concentration of aspecific gas (e.g., oxygen) contained in a gas under measurement and toperform another measurement such as measurement of the concentration ofH₂O gas contained in the gas under measurement by changing a targetvoltage, and which gas sensor apparatus can quickly stabilizefeedback-controlled pump current after the target voltage is changed.

The above object of the invention has been achieved by providing (1) agas sensor control apparatus for detecting the concentration of aspecific gas contained in a gas under measurement using a gas sensorwhich includes an electromotive force cell having an oxygen ionconductive first solid electrolyte body and a pair of first electrodesformed on the first solid electrolyte body, and a pump cell having anoxygen ion conductive second solid electrolyte body and a pair of secondelectrodes formed on the second solid electrolyte body, the gas sensorcontrol apparatus comprising: current control means forfeedback-controlling pump current flowing between the pair of secondelectrodes such that an electromotive force cell voltage producedbetween the pair of first electrode becomes equal to a target voltage;voltage setting means for setting the target voltage to either of afirst target voltage when the concentration of the specific gas isdetected and a second target voltage different from the first targetvoltage; and constant group setting means for setting a group of controlconstants used for the feedback control to a first group of controlconstants when the target voltage is the first target voltage and to asecond group of control constants when the target voltage is the secondtarget voltage, wherein at least one of the second group of controlconstants differs from a corresponding one of the first group of controlconstants; and the second group of control constants are determined suchthat when the pump current is feedback-controlled with the targetvoltage being switched from the first target voltage to the secondtarget voltage, the pump current becomes stable more quickly as comparedwith the case where the first group of control constants continues to beused.

This gas sensor control apparatus (1) includes constant group settingmeans for setting a group of control constants used for the feedbackcontrol to a first group of control constants or a second group ofcontrol constants. When the target voltage is switched from the firsttarget voltage to the second target voltage, the group of controlconstants are also switched from the first group of control constants tothe second group of control constants.

As a result, after the target voltage is switched to the second targetvoltage, the pump current can be stabilized more quickly as comparedwith the case where the feedback control is performed such that thefirst group of control constants continues to be used.

Examples of the specific gas whose concentration is detected by the gassensor include oxygen whose concentration is detected by an oxygensensor and nitrogen oxide (NOx) whose concentration is detected by anNOx sensor.

When the concentration of oxygen is detected, preferably, the firsttarget voltage is set to 400 mV to 500 mV. Examples of the feedbackcontrol used in the current control means include PI(proportion-integral) control and PID (proportional-integral-derivative)control.

Examples of the current control means which performs these controlsinclude an analog computation circuit which performs analog computation,and a microprocessor and a DSP (digital signal processor) which performdigital computation.

In a preferred embodiment (2) of the above-described gas sensor controlapparatus (1), the first target voltage is determined such that H₂O gascontained in the gas under measurement does not substantiallydissociate, and the second target voltage is higher than the firsttarget voltage and is determined such that the H₂O gas contained in thegas under measurement dissociates. The gas sensor control apparatusfurther comprises first current detection means for detecting, as afirst pump current, the pump current flowing between the pair of secondelectrodes in a state in which the electromotive force cell voltagebecomes equal to the first target voltage, second current detectionmeans for detecting, as a second pump current, the pump current flowingbetween the pair of second electrodes in a state in which theelectromotive force cell voltage becomes equal to the second targetvoltage, and H₂O concentration detection means for detecting theconcentration of the H₂O gas contained in the gas under measurement onthe basis of the first pump current and the second pump current.

In the gas sensor control apparatus (2), the concentration of the H₂Ogas contained in the gas under measurement is detected on the basis ofthe above-described first and second pump currents. In addition, sincethe group of control constants used for the feedback control is switchedsimultaneously with switching of the target voltage, the time requiredto obtain the second pump current after obtaining the first pump currentis short, and the second pump current can be obtained properly. As aresult, a gas sensor control apparatus can be obtained whose measurementtime is short and which can properly detect the H₂O gas concentration.

Notably, for detecting the H₂O gas concentration, a change in the oxygenconcentration of the gas under measurement which arises between the timeof measurement of the first pump current and the time of measurement ofthe second pump current is preferably eliminated. Therefore, preferably,the H₂O gas concentration is detected when the concentration of oxygencontained in the gas under measurement becomes a predetermined value;for example, when fuel cut is performed or when so-called stoichiometriccontrol is continuously performed, for example, during a period duringwhich a vehicle stops in an idling state.

The second target voltage is higher than the first target voltage and isdetermined such that the H₂O gas contained in the gas under measurementdissociates. In other words, the second target voltage must be increasedto a voltage at which the H₂O gas contained in the gas under measurementcan dissociate. However, when the second target voltage is increasedexcessively, the electromotive force cell (first solid electrolyte body)of the gas sensor may suffer blackening. Therefore, preferably, thesecond target voltage is determined to be as low as possible within arange within which the H₂O gas can dissociate to a sufficient degree.Specifically, the second target voltage is preferably set to a range of950 mV to 1100 mV.

In a preferred embodiment (3) of the above-described gas sensor controlapparatus (2), the H₂O concentration detection means detects the H₂O gasconcentration from a differential current obtained by subtracting thefirst pump current from the second pump current.

In the gas sensor control apparatus (3), since the H₂O gas concentrationis detected from the above-mentioned differential current, it ispossible to properly detect the H₂O gas concentration by simpleprocessing.

In another preferred embodiment (4) of any of the above-described gassensor control apparatuses (1) to (3), the current control meansincludes an analog computation circuit which performs analog computationfor the feedback control on the basis of the electromotive force cellvoltage; the analog computation circuit includes one or a plurality ofcircuit elements which determine the values of the group of controlconstants; and the constant group setting means includes a switch whichswitches the connection of the circuit elements of the analogcomputation circuit so as to set the group of control constants toeither of the first group of control constants and the second group ofcontrol constants.

In the gas sensor control apparatus (4), the group of control constantsused for the feedback control can be properly set by switching theconnection of the circuit elements by means of a switch.

Notably, example methods of switching the connection of the circuitelements by the switch include a method of switching the circuitelements to be used, a method of changing the way of connecting thecircuit elements together (e.g., from a series connection to a parallelconnection), and a method of forming a short circuit between oppositeends of each relevant circuit element or breaking the short circuit.

In yet another preferred embodiment (5) of any of the above-describedgas sensor control apparatuses (1) to (3), preferably, the currentcontrol means includes a computation section which performs digitalcomputation for the feedback control on the basis of the electromotiveforce cell voltage; and the constant group setting means sets the groupof control constants to either of the first group of control constantsand the second group of control constants.

According to the gas sensor control apparatus (5), the group of controlconstants can be set properly when the current control means performsdigital computation for the feedback control.

In yet another preferred embodiment (6) of any of the above-describedgas sensor control apparatuses (1) to (5), the feedback control is PIDcontrol; and the group of control constants includes at least one of aproportionality constant, an integration constant, and a differentiationconstant for the PID control.

According to the gas sensor control apparatus (6), the pump current canbe properly controlled under the feedback control realized by PIDcontrol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing an overall configuration of asystem in which a gas sensor and a gas sensor control apparatusaccording to an embodiment of the present invention are used for controlof an internal combustion engine.

FIG. 2 is an explanatory diagram schematically showing the configurationof the gas sensor control apparatus according to the embodiment.

FIG. 3 is a cross-sectional view schematically showing the structure ofthe gas sensor.

FIG. 4 is an explanatory diagram schematically showing the configurationof a PID control circuit according to the embodiment among the circuitsof the gas sensor control apparatus of FIG. 2.

FIG. 5 is a flowchart showing the processing operation of amicroprocessor of the gas sensor control apparatus according to theembodiment.

FIG. 6 is an explanatory diagram schematically showing the configurationof a PID control circuit according to a modification among the circuitsof the gas sensor control apparatus of FIG. 2.

DESCRIPTION OF REFERENCE NUMERALS AND SYMBOLS

Reference numerals and symbols used to identify various features in thedrawings include the following.

-   1, 1A: gas sensor control apparatus-   2: gas sensor-   3: sensor element section-   14: pump cell-   14 c: electrolyte layer (second electrolyte body)-   24: electromotive force cell-   24 c: electrolyte layer (first electrolyte body)-   12, 16: electrodes (of the pump cell) (second electrodes)-   22, 28: electrodes (of the electromotive force cell) (first    electrodes)-   Vs+, Ip+, COM: terminals (of the sensor element section)-   80: heater section-   Ip: pump current-   Vs: electromotive force cell voltage-   Vip: gas detection signal (oxygen concentration signal)-   30: microprocessor-   34: PWM output port-   40: sensor section control circuit-   59: control section (voltage setting means, constant group setting    means)-   61: differential amplification circuit (first current detection    means, second current detection means)-   69, 169: PID control circuit (current control means)-   69 a: first constant voltage source (voltage setting means)-   69 b: second constant voltage source (voltage setting means)-   69 e: PID computation section (analog computation circuit)-   69 f, 69 g, 69 h, 69 i, 69 j, 69 k: circuit element groups (circuit    elements)-   MUX1: analog multiplexer (voltage setting means)-   MUX2: analog multiplexer (constant group setting means)-   R1: detection resistor (first current detection means, second    current detection means)-   70: heater section control circuit-   ENG: internal combustion engine (engine)-   EP: exhaust pipe-   EG: exhaust gas (gas under measurement)-   100: ECU-   Vr: target voltage-   Vr1: first target voltage-   Vr2: second target voltage-   Ip1: first pump current-   Ip2: second pump current-   ΔIp: differential current-   CC: H₂O gas concentration-   Kpid: group of control constants-   Kpid1: first group of control constants-   Kpid2: second group of control constants-   S3: first current detection means-   S8: second current detection means-   S5, S13: voltage setting means-   S4, S12: constant group setting means-   S9, S10: H₂O concentration detection means

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will next be described withreference to the drawings. However, the present invention should not beconstrued as being limited thereto. FIG. 1 is a diagram showing anoverall configuration of a system in which a gas sensor controlapparatus 1 according to the present embodiment and a gas sensor 2 areused for control of an internal combustion engine. FIG. 2 is a diagramschematically showing the configuration of the gas sensor controlapparatus 1.

The gas sensor 2 is an air-fuel ratio sensor (full-range oxygen sensor)which is attached to an exhaust pipe EP of an internal combustion engineENG of a vehicle (not shown) and which linearly detects the oxygenconcentration (air-fuel ratio) of exhaust gas EG (gas under measurement)which is used for air-fuel ratio feedback control for the internalcombustion engine ENG. As shown in FIG. 2, this gas sensor 2 includes asensor element section 3 for detecting the oxygen concentration, and aheater section 80 for heating the sensor element section 3.

The gas sensor control apparatus 1 is connected to the gas sensor 2 andcontrols it. The gas sensor control apparatus 1 is also connected to aCAN (controller area network) bus 102 of the vehicle through aconnection bus 101, and can exchange data with an ECU (Engine ControlUnit) 100. The gas sensor control apparatus 1 includes a microprocessor30, a sensor section control circuit 40 for controlling the sensorelement section 3 of the gas sensor 2, and a heater section controlcircuit 70 for controlling the heater section 80.

First, the gas sensor 2 will be described. FIG. 3 is a viewschematically showing the structure of the gas sensor 2. The sensorelement section 3 of the gas sensor 2 is a layered sensor element formedby stacking a pump cell 14, a porous layer 18, and an electromotiveforce cell 24 in this sequence. The heater section 80 is further stackedon the sensor element section 3.

The pump cell 14 includes, as a substrate, an electrolyte layer 14 cwhich is composed of a platelike solid electrolyte body mainly made ofzirconia and having oxygen ion conductivity, and a pair of electrodes 12and 16 (porous electrodes) mainly made of platinum are formed onopposite sides of the electrolyte layer 14 c. Specifically, the outerelectrode 12 is formed on an outer surface 14E of the electrolyte layer14 c, which surface is one surface (upper surface in FIG. 3) of theelectrolyte layer 14 c, and the inner electrode 16 is formed on an innersurface 141 of the electrolyte layer 14 c, which surface is the othersurface (lower surface in FIG. 3) of the electrolyte layer 14 c.

Similarly, the electromotive force cell 24 includes, as a substrate, anelectrolyte layer 24 c which is composed of a platelike solidelectrolyte body mainly made of zirconia and having oxygen ionconductivity, and a pair of electrodes 22 and 28 (porous electrodes)mainly made of platinum are formed on opposite sides of the electrolytelayer 24 c. Specifically, the outer electrode 28 is formed on an outersurface 24E of the electrolyte layer 24 c, which surface is one surface(lower surface in FIG. 3) of the electrolyte layer 24 c, and the innerelectrode 22 is formed on an inner surface 241 of the electrolyte layer24 c, which surface is the other surface (upper surface in FIG. 3) ofthe electrolyte layer 24 c.

The inner surface 141 of the electrolyte layer 14 c of the pump cell 14faces the inner surface 241 of the electrolyte layer 24 c of theelectromotive force cell 24, and the porous layer 18 is sandwichedbetween the electrolyte layer 14 c and the electrolyte layer 24 c. Theporous layer 18 has a porous wall portion 18 c extending along the edgeof the inner surface 141 of the electrolyte layer 14 c and the edge ofthe inner surface 241 of the electrolyte layer 24 c, and the interior ofthe porous layer 18 forms a hollow measurement chamber 20 which issurrounded by the porous wall portion 18 c, the electrolyte layer 14 c,and the electrolyte layer 24 c and into which the exhaust gas EG can beintroduced. Notably, the porous layer 18 restricts the flow speed of theexhaust gas EG introduced into the measurement chamber 20.

The inner electrode 16 of the pump cell 14 and the inner electrode 22 ofthe electromotive force cell 24 are exposed to the measurement chamber20. These electrodes 16 and 22 are electrically connected together andare connected to a terminal COM of the sensor element section 3. Theouter electrode 12 of the pump cell 14 is connected to a terminal Ip+ ofthe sensor element section 3, and the outer electrode 28 of theelectromotive force cell 24 is connected to a terminal Vs+ of the sensorelement section 3.

The outer electrode 12 of the pump cell 14 is covered with a protectionlayer 15 for suppressing poisoning of the outer electrode 12. Theprotection layer 15 is formed of porous ceramic or the like and isdisposed in a flow passage through which the exhaust gas EG flows. Theexhaust gas EG can reach the outer electrode 12 through the protectionlayer 15.

The heater section 80 is stacked on the outer surface 24E of theelectrolyte layer 24 c of the electromotive force cell 24 and has astructure in which a heater resistor 87 made of a conductor issandwiched between a pair of alumina sheets 83 and 85. The electrolytelayers 14 c and 24 c of the sensor element section 3 are activated byincreasing the temperature of the sensor element section 3 by the heatersection 80. Thus, oxygen ions become able to move through theelectrolyte layers 14 c and 24 c.

The alumina sheet 83 of the heater section 80 covers the outer electrode28 of the electromotive force cell 24 to thereby seal the outerelectrode 28. Notably, spaces (holes) within the outer electrode 28(porous electrode) form a reference oxygen chamber 26, and function asan internal oxygen reference source as described below.

Next, the gas sensor control apparatus 1 will be described withreference to FIG. 2. The sensor section control circuit 40 is mainlyconstituted by an ASIC (application-specific integrated circuit), and isconnected to the three terminals Vs+, Ip+, COM of the sensor elementsection 3 via connection paths 41, 42, 43 (specifically, wiring lines onthe circuit board and lead wires). While supplying a fixed very smallcurrent Icp to the electromotive force cell 24 of the sensor elementsection 3, the sensor section control circuit 40 controls the pumpcurrent Ip flowing through the pump cell 14 such that the electromotiveforce cell voltage Vs generated between the opposite ends of theelectromotive force cell 24 becomes 450 mV (=a first target voltage Vr1,described below), to thereby pump out oxygen contained in the exhaustgas EG introduced into the measurement chamber 20 through the porouslayer 18 or pump oxygen into the measurement chamber 20. Since themagnitude and flow direction of the pump current Ip flowing through thepump cell 14 change with the oxygen concentration (air-fuel ratio) ofthe exhaust gas EG introduced into the measurement chamber 20 throughthe porous layer 18, the concentration of oxygen contained in theexhaust gas EG can be detected on the basis of the pump cell current Ip.Notably, the very small current Icp flows through the electromotiveforce cell 24 in such a direction that the oxygen within the measurementchamber 20 is pumped out to the outer electrode 28 (porous electrode).Therefore, the reference oxygen chamber 26 functions as an internaloxygen reference source.

In the sensor section control circuit 40, the magnitude of the pumpcurrent Ip is converted to a voltage signal, which is output from a gasdetection signal output terminal 44 as a gas detection signal Vip. Also,in addition to the gas detection signal Vip, the sensor section controlcircuit 40 detects a voltage change amount ΔVs which changes inaccordance with the element resistance Rpvs of the electromotive forcecell 24 of the sensor element section 3, and outputs ΔVs from a voltagechange amount output terminal 45. The microprocessor 30 can receive thegas detection signal Vip and the voltage change amount ΔVs via A/D inputports 31, 32. Notably, the value of the detected gas detection signalVip is sent to the ECU 100 through the connection bus 101.

The heater section control circuit 70 is connected to the heater section80 of the gas sensor 2 via two lead wires 71, 72, and is connected to aPWM (pulse-width-modulated) output port 34 of the microprocessor 30. Theheater section control circuit 70 supplies electric current to theheater section 80, through PWM control, in accordance with PWM pulsesoutput from the PWM output port 34.

Next, operation of the sensor section control circuit 40 for measuringthe oxygen concentration using the sensor element section 3 will bedescribed.

The terminal COM of the sensor element section 3 is connected to a Vcentpoint via the connection path 43 and a resistor R. The terminal Ip+ isconnected to the output terminal of a second operational amplifier OP2via the connection path 42. The terminal Vs+ is connected to thenoninverting input terminal+of a fourth operational amplifier OP4 viathe connection path 41. The terminal Vs+ is also connected to a constantcurrent source circuit 62. This constant current source circuit 62supplies the above-described fixed very small current Icp to theelectromotive force cell 24.

The sensor section control circuit 40 is composed of first through fifthoperational amplifiers OP1-OP5, one first switch SW1, three secondswitches SW2, two third switches SW3, a PID control circuit 69, adifferential amplification circuit 61, current sources 63, 64, 65, 66, acontrol section 59, etc., as well as the above-mentioned resistor R andthe constant current source circuit 62. The constant current sourcecircuit 62, the electromotive force cell 24, and the resistor R areconnected in this order through the connection paths 41, 43 to therebyform a current path through which the very small current Icp is causedto flow.

When the oxygen concentration is measured, the first switch SW1 isbrought into the ON state by the control section 59. As a result, thepotential at the terminal Vs+ of the sensor element section 3 is inputto the input terminal IT of the PID control circuit 69 via theconnection path 41 and the fourth operational amplifier OP4 and thefirst operational amplifier OP1, which form a voltage follower circuit.The control section 59 is a logic circuit formed within the ASIC, whichconstitutes the sensor section control circuit 40. The control section59 is connected to a serial transmission port 33 of the microprocessor30 via a command reception port 46 of the sensor section control circuit40. In response to instructions from the microprocessor 30, the controlsection 59 controls the ON/OFF states of the first through thirdswitches SW1-SW3 and performs other controls.

One input terminal of the second operational amplifier OP2 is connectedto the Vcent point, and a reference voltage Vc (=+3.6 V) is applied tothe other input terminal of the second operational amplifier OP2. Asdescribed above, the output terminal of the second operational amplifierOP2 is connected to the terminal Ip+ of the sensor element section 3 viathe connection path 42. Notably, the Vcent point is also connected tothe reference terminal RT of the PID control circuit 69.

The PID control circuit 69 has an output terminal OT in addition to theabove-described input terminal IT and reference terminal RT. The PIDcontrol circuit 69 controls the magnitude of the pump current Ip bymeans of PID control such that a voltage difference, that is producedbetween the potential at the Vcent point (reference terminal RT) and thepotential at the terminal Vs+ of the sensor element section 3 (inputterminal IT) which is input via the fourth operational amplifier OP4 andthe first operational amplifier OP1, becomes 450 mV. Specifically, thePID control circuit 69 calculates, through PID computation, thedifference between a control target voltage (450 mV) and theelectromotive force cell voltage Vs generated between the opposite endsof the electromotive force cell 24 (between the electrodes 28, 22), andfeeds the difference back to the second operational amplifier OP2. Thus,the second operational amplifier OP2 supplies the pump current Ip to thepump cell 14.

Moreover, the sensor section control circuit 40 includes a detectionresistor R1, which detects the magnitude of the pump current Ip, andconverts it to a voltage signal. The voltage generated across thedetection resistor R1 (the differential voltage between potentials Vcentand Vpid) is differentially amplified by the differential amplificationcircuit 61, and is output from the gas detection signal output terminal44 as the gas detection signal Vip. Since the magnitude and direction ofthe pump current Ip change in accordance with the oxygen concentration(air-fuel ratio) as described above, the oxygen concentration can bedetected from the gas detection signal Vip, which is a voltage signalrepresenting the magnitude of the pump current Ip.

The microprocessor 30 obtains the gas detection signal (oxygenconcentration signal) Vip, in the form of a digital signal, through theA/D input port 31 (i.e., through A/D conversion), and sends the obtainedvalue to the ECU 100 via the connection bus 101.

Notably, the sensor section control circuit 40 is also used to detect avoltage change amount ΔVs which changes in accordance with the elementresistance Rpvs of the electromotive force cell 24. The sensor sectioncontrol circuit 40 shown in FIG. 2 includes a circuit block used fordetection of the voltage change amount ΔVs. However, since the operationof the circuit block and the detail of the circuit block are disclosedin, for example, Japanese Patent Application Laid-Open (kokai) No.2008-203190 and U.S. Pat. No. 6,120,677, incorporated herein byreference, and are well known, its description is omitted here. Asdescribed above, the voltage change amount ΔVs is output from thevoltage change amount output terminal 45 of the sensor section controlcircuit 40 and is input to the A/D input port 32 of the microprocessor30. The microprocessor 30 regularly detects the value of the voltagechange amount ΔVs and detects the element resistance Rpvs of theelectromotive force cell 24 from the voltage change amount ΔVs. Thesupply of electric current to the heater section 80 isfeedback-controlled by the heater section control circuit 70 such thatthe element resistance Rpvs becomes equal to a predetermined targetresistance, whereby the sensor element section 3 is heated.

Next, a method of measuring the H₂O gas concentration will be describedusing the gas sensor control apparatus 1. As described above, when theconcentration of oxygen contained in the exhaust gas EG is measured, thepump current Ip is controlled such that the electromotive force cellvoltage Vs becomes equal to a first target voltage Vr1 (=450 mV). Incontrast, when the H₂O gas concentration is measured, the pump currentIp is controlled such that the electromotive force cell voltage Vsbecomes equal to a second target voltage Vr2 (=1000 mV). Namely, theelectromotive force cell voltage Vs is switched from the first targetvoltage Vr1 (=450 mV) to the second target voltage Vr2 (=1000 mV).Specifically, as shown in FIG. 4, the PID control circuit 69 of thesensor section control circuit 40 includes a first constant voltagesource 69 a which outputs a first set value V1 (=Vc−Vr1) set such thatthe target voltage Vr of the electromotive force cell voltage Vs becomesthe first target voltage Vr1 (=450 mV), and a second constant voltagesource 69 b which outputs a second set value V2 (=Vc−Vr2) set such thatthe target voltage Vr of the electromotive force cell voltage Vs becomesthe second target voltage Vr2 (=1000 mV). The PID control circuit 69 isconfigured to switch the electromotive force cell voltage Vs between thefirst and second target voltages Vr1 and Vr2. More specifically, one ofthe outputs of the first constant voltage source 69 a and the secondconstant voltage source 69 b is input to the noninverting inputterminal+of an operational amplifier 69 c as a result of switching of ananalog multiplexer MUX1 by the control section 59. The output of theoperational amplifier 69 c, which forms a voltage follower, becomesequal to the first set voltage V1 (=Vc−Vr1) or the second set voltage V2(=Vc−Vr2). In addition, the PID control circuit 69 includes anoperational amplifier 69 d which amplifies the difference between thevoltage at the reference terminal RT (≈Vc) and the sum of the voltage atthe input terminal IT (=Vs+) and the first set voltage V1 (=Vc−Vr1) orthe second set voltage V2 (=Vc−Vr2), and a PID computation section 69 ewhich is connected to the output of the operational amplifier 69 d andwhich performs PID computation.

Furthermore, an analog multiplexer MUX2 and circuit element groups 69 fto 69 k are externally connected to the PID computation section 69 e ofthe present embodiment so as to switch the group of control constantsused for feedback control of the pump current Ip. The analog multiplexerMUX2 and the circuit element groups 69 f to 69 k will be described indetail below. By means of these components, the pump current Ip iscontrolled such that the electromotive force cell voltage Vs becomesequal to the target voltage Vr which is switched between the firsttarget voltage Vr1 (=450 mV) and the second target voltage Vr2 (=1000mV).

The second target voltage Vr2 (=1000 mV) is a voltage determined suchthat not only the O₂ gas contained in the exhaust gas EG but also theH₂O gas contained in the exhaust gas EG dissociates. Meanwhile, thefirst target voltage Vr1 (=450 mV) is a voltage determined such that theH₂O gas contained in the exhaust gas EG does not substantiallydissociate although the O₂ gas contained in the exhaust gas EGdissociates.

The ECU 100 issues an instruction for instructing measurement of the H₂Ogas concentration when a predetermined measurement start condition issatisfied; for example, when the state of supply of the exhaust gas EG(gas under measurement) to the gas sensor 2 is a state in which theexhaust gas EG continuously has a predetermined oxygen concentration(for example, when fuel cut is performed or so-called stoichiometriccontrol is continuously performed during a period during which thevehicle stops in an idling state because, for example, the driver of thevehicle waits for a traffic light to change). The gas sensor controlapparatus 1 of the present embodiment starts a measurement operation inresponse to this instruction. Immediately after the start of themeasurement operation, the pump current Ip is controlled such that theelectromotive force cell voltage Vs becomes equal to the first targetvoltage Vr1 because the target voltage Vr is set to the first targetvoltage Vr1 (=450 mV). Subsequently, in a state in which theelectromotive force cell voltage Vs has become equal to the first targetvoltage Vr1, the pump current Ip flowing between the electrodes 12 and16 is measured as a first pump current Ip1. Notably, as described above,the magnitude of the pump current Ip is detected by the detectionresistor R1, and is output as the gas detection signal (oxygenconcentration signal) Vip.

Next, the target voltage Vr is switched to the second target voltage Vr2(=1000 mV), and the pump current Ip is controlled such that theelectromotive force cell voltage Vs becomes equal to the second targetvoltage Vr2. In a state in which the electromotive force cell voltage Vsbecomes equal to the second target voltage Vr2 after elapse of apredetermined period of time required for the electromotive force cellvoltage Vs and the pump current Ip to become stable, the pump current Ipflowing between the electrodes 12 and 16 is measured as the second pumpcurrent Ip2. Notably, it is assumed that during this measurement, theoxygen concentration of the exhaust gas EG is constant; for example, thestate of supply of the exhaust gas EG; i.e., the drive state of theengine (e.g., idling state) is maintained constant, whereby the air-fuelratio is maintained at the stoichiometric air-fuel ratio.

After that, a differential current ΔIp (=Ip2−Ip1) is calculated bysubtracting the first pump current Ip1 which flows as a result ofdissociation of the O₂ gas from the second pump current Ip2. Since thisdifferential current ΔIp is a current attributable to the dissociationof the H₂O gas, the concentration of the H₂O gas contained in theexhaust gas EG can be determined from the differential current ΔIp.

However, after the target voltage Vr is switched from the first voltageVr1 (=450 mV) to the second voltage Vr2 (=1000 mV), it is necessary towait a relatively long time (e.g., several to about 10 seconds) untilthe feedback-controlled pump current Ip becomes stable and it becomespossible to obtain a proper second pump current Ip2. Meanwhile, during aperiod during which the H₂O gas concentration is detected, the oxygenconcentration cannot be detected. Therefore, in engine control, feedbackcontrol of the air-fuel ratio which is performed using the oxygenconcentration output (air-fuel ratio output) of the gas sensor cannot beperformed during this period, and the air-fuel ratio is subject toopen-loop control. Therefore, there is a need to shorten, to the extentpossible, the time required for properly detecting the H₂O gasconcentration.

In view of this, in the gas sensor control apparatus 1 of the presentembodiment, when the target voltage Vr is switched from the firstvoltage Vr1 (=450 mV) to the second voltage Vr2 (=1000 mV), the group ofcontrol constants used for the feedback control of the pump current Ipis switched in the PID computation section 69 e of the PID controlcircuit 69 so as to quickly stabilize the pump current Ip. As shown inFIG. 4, the analog multiplexer MUX2 and the circuit element groups 69 fto 69 k each including a resistor and a capacitor (not shown) areexternally connected to the PID computation section 69 e of the PIDcontrol circuit 69. The PID computation section 69 e is mainly composedof an unillustrated operational amplifier and forms, together with theexternal circuit element groups 69 f to 69 k, an analog computationcircuit which performs analog computation for PID feedback control.

The circuit element groups 69 f, 69 g, and 69 h set the values of thecontrol constants used for the PID computation performed in the PIDcomputation section 69 e (i.e., a proportionality constant Kp, anintegration constant Ki, and a differentiation constant Kd) to Kp1, Ki1,and Kd1 (hereinafter these constants will be referred to as the “firstgroup of control constants Kpid1”). Meanwhile, the circuit elementgroups 69 i, 69 j, and 69 k set the values of the control constants usedfor the PID computation performed in the PID computation section 69 e(i.e., the proportionality constant Kp, the integration constant Ki, andthe differentiation constant Kd) to Kp2, Ki2, and Kd2 (hereinafter theseconstants will be referred to as the “second group of control constantsKpid2”), which differ from Kp1, Ki1, and Kd1, respectively. The circuitelement group 69 f and the circuit element group 69 i, the circuitelement group 69 g and the circuit element group 69 j, and the circuitelement group 69 h and the circuit element group 69 k form respectivepairs and are connected to the analog multiplexer MUX2. As a result ofthe internal switches of the analog multiplexer MUX2 beingsimultaneously switched by the control section 59, the first group ofcontrol constants Kpid1 or the second group of control constants Kpid2are used as the group of control constants Kpid in the PID controlcircuit 69. Notably, when the second group of control constants Kpid2are used as the group of control constants Kpid, the pump current Ipbecomes stable more quickly as compared with the case where the firstgroup of control constants Kpid1 continues to be used. Therefore, in thepresent embodiment, when the target voltage Vr is the first targetvoltage Vr1, the group of control constants Kpid are set to the firstgroup of control constants Kpid1, and when the target voltage Vr is thesecond target voltage Vr2, the group of control constants Kpid are setto the second group of control constants Kpid2.

Notably, in the present embodiment, the values of Kp2, Ki2, and Kd2 ofthe second group of control constants Kpid2 differ from the values ofKp1, Ki1, and Kd1 of the first group of control constants Kpid1.However, the values of Kp2, Ki2, and Kd2 of the second group of controlconstants Kpid2 may be freely determined so long as when the pumpcurrent Ip is feedback-controlled with the target voltage Vr beingswitched from the first target voltage Vr1 to the second target voltageVr2, the pump current Ip becomes stable more quickly as compared withthe case where the first group of control constants Kpid1 continues tobe used. In order to satisfy this requirement, it is sufficient for oneof the control constants, i.e., the proportionality constant Kp, theintegration constant Ki, and the differentiation constant Kd, of thesecond group of control constants Kpid2 to differ from the first groupof control constants Kpid1.

As stated in the specification, examples of the feedback control used inthe current control means include PI (proportion-integral) control andPID (proportional-integral-derivative) control. When switching the groupof control constants from the first group of control constants to thesecond group of control constants, each constant value of proportion(P), integration (I) and derivative (D) can be appropriately changed inconsideration of frequency properties of a gas sensor.

Examples of the second group of control constants may include a valuethat any one of the constants forming the first group of controlconstants is increased. For example, when the feedback control is PIDcontrol, at least a value of any one of P, I, D constants (moreparticularly, proportionality constant) is made large in order to formthe second group of control constants.

Alternatively, when the feedback control is PID control, the secondgroup of control constants may be formed such that values ofproportionality constant and differentiation constant in P, I, Dconstants forming the first group of control constants are made large,while a value of integral constant is made small.

As described above, in the gas sensor control apparatus 1 of the presentembodiment, when the H₂O gas concentration is detected, simultaneouslywith the switching of the target voltage Vr from the first targetvoltage Vr1 to the second target voltage Vr2, the group of controlconstants Kpid of the PID control circuit 69 (PID computation section 69e) used for the feedback control of the pump current Ip are switchedfrom the first group of control constants Kpid1 to the second group ofcontrol constants Kpid2.

As a result, after the target voltage Vr is switched to the secondtarget voltage Vr2, the pump current Ip can be stabilized more quicklyas compared with the case where the first group of control constantsKpid1 continues to be used.

Next, operation of the gas sensor control apparatus 1 (particularly, themicroprocessor 30) according to the present embodiment will be describedwith reference to the flowchart of FIG. 5.

The microprocessor 30 of the gas sensor control apparatus 1 periodicallyperforms the main processing shown in FIG. 5 every time a predeterminedtiming comes so as to detect the oxygen concentration. When detection ofthe H₂O gas concentration is instructed by the ECU 100, themicroprocessor 30 performs the processing for detecting the H₂O gasconcentration.

When the microprocessor 30 starts the main processing upon arrival ofthe predetermined timing, the microprocessor 30 first detects the oxygenconcentration in step S1. Notably, at this time, in the PID controlcircuit 69 of the sensor section control circuit 40, the target voltageVr of the electromotive force cell voltage Vs is the first targetvoltage Vr1 (=450 mV). Therefore, the pump current Ip is controlled suchthat the electromotive force cell voltage Vs becomes equal to the firsttarget voltage Vr1. Also, the group of control constants Kpid of the PIDcontrol circuit 69 (PID computation section 69 e) used for the feedbackcontrol of the pump current Ip is set to the first group of controlconstants Kpid1. The magnitude of the pump current Ip at this time isdetected as the gas detection signal (oxygen concentration signal) Vip.

In subsequent step S2, the microprocessor 30 determines whether or notit receives a detection instruction which instructs detection of the H₂Ogas concentration and which is output from the ECU 100 when thepredetermined measurement start condition is satisfied (e.g., when fuelcut is performed or the vehicle stops in an idling state). In the casewhere the microprocessor 30 does not receive the detection instruction(No), the microprocessor 30 ends the present main processing withoutperforming any processing. Namely, the microprocessor 30 performs onlydetection of the oxygen concentration in step S1. Meanwhile, in the casewhere the microprocessor 30 receives the detection instruction whichinstructs detection of the H₂O gas concentration (Yes), themicroprocessor 30 proceeds to step S3.

In step S3, the microprocessor 30 sets the target voltage Vr to thefirst target voltage Vr1 (=450 mV), reads, as the first pump currentIp1, the magnitude of the pump current Ip (gas detection signal Vip) atthe time when the electromotive force cell voltage Vs becomes equal tothe first target voltage Vr1, and stores the first pump current Ip1.Next, in step S4, the microprocessor 30 switches the group of controlconstants Kpid from the first group of control constants Kpid1 to thesecond group of control constants Kpid2 before changing the targetvoltage Vr in step S5. Specifically, the microprocessor 30 switches theanalog multiplexer MUX2 through the control section 59 such that inplace of the circuit element groups 69 f, 69 g, and 69 h, the circuitelement groups 69 i, 69 j, and 69 k are used in the PID computationsection 69 e. Next, in step S5, the microprocessor 30 switches thetarget voltage Vr to the second target voltage Vr2 (=1000 mV).Specifically, the microprocessor 30 switches the analog multiplexer MUX1through the control section 59 such that in place of the first constantvoltage source 69 a, the second constant voltage source 69 b isconnected to the operational amplifier 69 c. In step S6 subsequentthereto, the microprocessor 30 determines whether or not a predeterminedperiod of time has elapsed after switching of the target voltage Vr instep S5. The predetermined period of time is a period of time requiredfor the electromotive force cell voltage Vs and the pump current Ip tobecome stable. In the case where the predetermined period of time hasnot yet elapsed (No), the microprocessor 30 proceeds to step S7.

In step S7, the microprocessor 30 determines whether or not the H₂O gasconcentration detection instruction output from the ECU 100 in step S2still continues. In the case where the detection instruction continues(Yes), the microprocessor 30 returns to step S6 and waits for the elapseof the predetermined period of time while repeating steps S6 and S7. Inthe case where the predetermined period of time has elapsed, themicroprocessor 30 makes a “Yes” determination in step S6 and proceeds tostep S8. Meanwhile, in the case where the detection instruction from theECU 100 is discontinued while waiting for the elapse of thepredetermined period of time, the microprocessor 30 makes a “No”determination in step S7 and proceeds to step S12.

Notably, as a result of switching of the group of control constants Kpidto the second group of control constants Kpid2 in step S4, the pumpcurrent Ip becomes stable more quickly as compared with the case wherethe feedback control is performed while the first group of controlconstants Kpid1 continues to be used independent of the target voltage.Therefore, the predetermined period of time in step S6 is set to beshorter than the case where the first group of control constants Kpid1continues to be used.

In step S8, the microprocessor 30 reads, as the second pump current Ip2,the magnitude of the pump current Ip (gas detection signal Vip) at thetime when the electromotive force cell voltage Vs becomes equal to thesecond target voltage Vr2 (=1000 mV) and stores the second pump currentIp2.

In step S9 subsequent thereto, the microprocessor 30 calculates thedifferential current ΔIp (ΔIp=Ip2−Ip1). In step S10 subsequent thereto,the microprocessor 30 obtains a H₂O gas concentration CC correspondingto the obtained differential current ΔIp with reference to a table whichshows the relation between the differential current ΔIp and the H₂O gasconcentration CC. In step S11 subsequent thereto, the microprocessor 30sends the detected H₂O gas concentration CC to the ECU 100 via theconnection bus 101.

Next, in step S12, the microprocessor 30 returns the group of controlconstants Kpid to the first group of control constants Kpid1.Specifically, the microprocessor 30 switches the analog multiplexer MUX2through the control section 59 such that in place of the circuit elementgroups 69 i, 69 j, and 69 k, the circuit element groups 69 f, 69 g, and69 h are used in the PID computation section 69 e. Furthermore, in stepS13 subsequent thereto, the microprocessor 30 returns the target voltageVr to the first target voltage Vr1 (=450 mV). Specifically, themicroprocessor 30 switches the analog multiplexer MUX1 through thecontrol section 59 such that in place of the second constant voltagesource 69 b, the first constant voltage source 69 a is connected to theoperational amplifier 69 c. After completing step S13, themicroprocessor 30 ends the present main processing. Notably, when themicroprocessor 30 makes a “No” determination in step S7, themicroprocessor 30 ends the present main processing after executing stepsS12 and S13.

In the present embodiment, the electrodes 22 and 28 of the electromotiveforce cell 24 correspond to the pair of first electrodes, and theelectrodes 12 and 16 of the pump cell 14 correspond to the pair ofsecond electrodes of the invention. The electrolyte layer 24 c of theelectromotive force cell 24 corresponds to the first solid electrolytebody, and the electrolyte layer 14 c of the pump cell 14 corresponds tothe second solid electrolyte body of the invention.

The PID control circuit 69 of the sensor section control circuit 40corresponds to the current control means of the invention. The detectionresistor R1 of the sensor section control circuit 40, the differentialamplification circuit 61 of the sensor section control circuit 40, andthe microprocessor 30 which executes step S3 correspond to the firstcurrent detection means of the invention. The detection resistor R1 ofthe sensor section control circuit 40, the differential amplificationcircuit 61 of the sensor section control circuit 40, and themicroprocessor 30 which executes step S8 correspond to the secondcurrent detection means of the invention. The first constant voltagesource 69 a, the second constant voltage source 69 b, and the analogmultiplexer MUX1 of the PID control circuit 69 of the sensor sectioncontrol circuit 40, the control section 59, and the microprocessor 30which executes steps S5 and S13 correspond to the voltage setting meansof the invention.

The PID computation section 69 e of the PID control circuit 69corresponds to the analog computation circuit of the invention whichperforms analog computation for feedback control. The analog multiplexerMUX2 corresponds to the switch of the invention for switching theconnection of the circuit element groups 69 f to 69 k. The analogmultiplexer MUX2, the control section 59, and the microprocessor 30which executes steps S4 and S12 correspond to the constant group settingmeans of the invention.

The microprocessor 30 which executes steps S9 and S10 corresponds to theH₂O gas concentration detection means of the invention.

As described above, the gas sensor control apparatus 1 of the presentembodiment includes the constant group setting means (step S4, S12)which sets the group of control constants Kpid used for feedback controlof the pump current Ip to the first group of control constants Kpid1 orthe second group of control constants Kpid2. When the target voltage Vris switched from the first target voltage Vr1 to the second targetvoltage (step S5), the group of control constants Kpid are also switchedfrom the first group of control constants Kpid1 to the second group ofcontrol constants Kpid2 (step S4).

As a result, after the target voltage Vr is switched to the secondtarget voltage Vr2, the pump current Ip can be stabilized more quicklyas compared with the case where the first group of control constantsKpid1 continues to be used. Namely, as a result of switching from thefirst to the second group of control constants, the predetermined periodof time in step S6 can be set to a shorter period of time.

In the gas sensor control apparatus 1 of the present embodiment, the H₂Ogas concentration of the gas under measurement is detected on the basisof the first pump current Ip1 and the second pump current Ip2. Inaddition, since the group of control constants Kpid used for feedbackcontrol of the pump current Ip are changed simultaneously with thechanging of the target voltage Vr, the time required to obtain thesecond pump current Ip2 after obtaining the first pump current Ip1 isshort, and the second pump current Ip2 can be obtained properly. As aresult, a gas sensor control apparatus 1 can be obtained whosemeasurement time is short and which can properly detect the H₂O gasconcentration.

In the gas sensor control apparatus 1 of the present embodiment, sincethe H₂O gas concentration is detected using the differential current ΔIp(=Ip2−Ip1), the H₂O gas concentration can be properly detected throughsimple processing.

In the gas sensor control apparatus 1 of the present embodiment, thegroup of control constants Kpid used for feedback control can beproperly switched between the first group of control constants Kpid1 andthe second group of control constants Kpid2 by switching the connectionof the circuit element groups 69 f to 69 k by the analog multiplexerMUX2.

According to the gas sensor control apparatus 1 of the presentembodiment, the pump current Ip can be properly controlled underfeedback control realized by PID control of the PID control circuit 69.

(Modification)

Next, a gas sensor control apparatus 1A according to a modification ofthe above-described embodiment will be described. In the gas sensorcontrol apparatus 1 of the above-described embodiment, the PID controlcircuit 69 of the sensor section control circuit 40 includes a PIDcomputation section 69 e, which is an analog computation circuit, asshown in FIG. 4, and the switching between the first group of controlconstants Kpid1 and the second group of control constants Kpid2 isperformed by switching the connection of the circuit element groups 69 fto 69 k by the analog multiplexer MUX2.

In contrast, in the gas sensor control apparatus 1A of the presentmodification, the sensor section control circuit 40 includes a PIDcontrol circuit 169 in place of the PID control circuit 69 formed by ananalog computation circuit. The PID control circuit 169 is formed by aDSP (digital signal processor) which includes a digital computationsection 169 b shown in FIG. 6. Namely, in the present modification, anASIC which includes a DSP for forming the PID control circuit 169 isused for the sensor section control circuit 40.

The PID control circuit 169 includes an A/D conversion section 169 a anda D/A conversion section 169 c in addition to the digital computationsection 169 b. The potentials at the reference terminal RT and the inputterminal IT are converted to digital values by the A/D converter 169 aand are input to the digital computation section 169 b. The digitalcomputation section 169 b includes a voltage setting section 169 b 1which sets, as the target voltage Vr, a digital value corresponding toeither of the first target voltage Vr1 and the second target voltage Vr2in response to an instruction from the control section 59; and aconstant group setting section 169 b 2 which sets, as the group ofcontrol constants Kpid, the digital values corresponding to either ofthe first group of control constants Kpid1 and the second group ofcontrol constants Kpid2 in response to an instruction from the controlsection 59. The digital computation section 169 b performs digitalcomputation for the feedback control (PID control) by using thesedigital values, and outputs the result of the computation (in the formof a digital value) to the D/A conversion section 169 c. The D/Aconversion section 169 c converts the digital value output from thedigital computation section 169 b to an analog voltage, and outputs itfrom the output terminal OT. The portions of the sensor section controlcircuit 40 other than the PID control circuit 169 perform the sameprocessing operations as those in the embodiment shown in FIG. 2. Theremaining portion has the same configuration as that of the embodiment,and the microprocessor 30 performs the same operation as that of theembodiment shown by the flowchart of FIG. 5. Therefore, theirdescriptions are omitted.

In the present modification, the PID control circuit 169 corresponds tothe current control means of the invention. The voltage setting section169 b 1, the control section 59, and the microprocessor 30 whichexecutes steps S5 and S13 correspond to the voltage setting means of theinvention.

The digital computation section 169 b of the PID control circuit 169corresponds to the computation section of the invention which performsdigital computation for feedback control. The constant group settingsection 169 b 2, the control section 59, and the microprocessor 30 whichexecutes steps S4 and S12 correspond to the constant group setting meansof the invention.

In the gas sensor control apparatus 1A of the present modification, theconstant group setting section 169 b 2 is provided in the digitalcomputation section 169 b, and as in the case of the embodiment, thegroup of control constants Kpid is also switched from the first group ofcontrol constants Kpid1 to the second group of control constants Kpid2when the target voltage Vr is switched from the first target voltage Vr1to the second target voltage Vr2.

As a result, after the target voltage Vr is switched to the secondtarget voltage Vr2, the pump current Ip can be stabilized more quicklyas compared with the case where the first group of control constantsKpid1 continues to be used.

According to the gas sensor control apparatus 1A of the presentmodification, when digital computation for feedback control isperformed, the group of control constants Kpid can be properly set bythe constant group setting section 169 b 2 of the digital computationsection 169 b.

According to the gas sensor control apparatus 1A of the presentmodification, the pump current Ip can be properly controlled underfeedback control realized by PID control of the PID control circuit 169.

In the above, the gas sensor control apparatus of the present inventionhas been described on the basis of the gas sensor control apparatus 1according to the embodiment and the gas sensor control apparatus 1Aaccording to the modification. However, needless to say, the presentinvention is not limited to the embodiment and modification, and can bemodified freely without departing from the scope of the invention.

For example, in the embodiment and the modification, the gas sensor 2 isan oxygen sensor for detecting the oxygen concentration (air-fuel ratio)of exhaust gas EG. However, the “gas sensor” is not limited to theoxygen sensor, and may be an NOx sensor for detecting the concentrationof nitrogen oxide (NOx), or the like.

Also, the gas sensor is not limited to those attached to the exhaustpipe, and the present invention may be applied to a gas sensor which isattached to the intake pipe of an engine having an EGR device and whichdetects the concentration of oxygen contained in intake gas.

In the embodiment, all of the proportionality constant Kp, theintegration constant Ki, and the differentiation constant Kd areswitched. However, only a portion of the group of control constants maybe switched. For example, of the control constants, only theproportionality constant Kp may be switched or only the proportionalityconstant Kp and the integration constant Ki may be switched. Also, theembodiment may be modified such that control means other than PI controland PID control is employed and the control constants used by thecontrol means are switched.

In the embodiment, the circuit element groups (69 f and 69 i, 69 g and69 j, 69 h and 69 k) to be used are switched by the analog multiplexerMUX2. However, the embodiment may be configured such that the way ofconnection between circuit elements connected together is changed, or ashort circuit is formed between opposite ends of each relevant circuitelement or broken.

In the modification, an ASIC which includes a DSP for forming the PIDcontrol circuit 169 is used for the sensor section control circuit 40,and digital computation for feedback control is performed by the DSPincorporated in the ASIC. However, such digital computation may beperformed by a microprocessor or a dedicated digital computation circuitwhich is provided separately from the ASIC.

The invention has been described in detail with reference to the aboveembodiments. However, the invention should not be construed as beinglimited thereto. It should further be apparent to those skilled in theart that various changes in form and detail of the invention as shownand described above may be made. It is intended that such changes beincluded within the spirit and scope of the claims appended hereto.

This application is based on Japanese Patent Application No. 2012-125403filed May 31, 2012, incorporated herein by reference in its entirety.

What is claimed is:
 1. A gas sensor control apparatus for detecting theconcentration of a specific gas contained in a gas under measurementusing a gas sensor which includes an electromotive force cell having anoxygen ion conductive first solid electrolyte body and a pair of firstelectrodes formed on the first solid electrolyte body, and a pump cellhaving an oxygen ion conductive second solid electrolyte body and a pairof second electrodes formed on the second solid electrolyte body, thegas sensor control apparatus comprising: current control means forfeedback-controlling pump current flowing between the pair of secondelectrodes such that an electromotive force cell voltage producedbetween the pair of first electrode becomes equal to a target voltage;voltage setting means for setting the target voltage to either of afirst target voltage when the concentration of the specific gas isdetected and a second target voltage different from the first targetvoltage; and constant group setting means for setting a group of controlconstants used for the feedback control to a first group of controlconstants when the target voltage is the first target voltage and to asecond group of control constants when the target voltage is the secondtarget voltage, wherein at least one of the second group of controlconstants differs from a corresponding one of the first group of controlconstants; the second group of control constants are determined suchthat when the pump current is feedback-controlled with the targetvoltage being switched from the first target voltage to the secondtarget voltage, the pump current becomes stable more quickly as comparedwith the case where the first group of control constants continues to beused; the constant group setting means switches the control constants tothe second group from the first group before the voltage setting meanschanges the target voltage to the second target voltage from the firsttarget voltage; the first target voltage is determined such that H₂O gascontained in the gas under measurement does not substantiallydissociate; the second target voltage is higher than the first targetvoltage and is determined such that the H₂O gas contained in the gasunder measurement dissociates; and the gas sensor control apparatusfurther comprises: first current detection means for detecting, as afirst pump current, the pump current flowing between the pair of secondelectrodes in a state in which the electromotive force cell voltagebecomes equal to the first target voltage, second current detectionmeans for detecting, as a second pump current, the pump current flowingbetween the pair of second electrodes in a state in which theelectromotive force cell voltage becomes equal to the second targetvoltage, and H₂O concentration detection means for detecting theconcentration of the H₂O gas contained in the gas under measurement onthe basis of the first pump current and the second pump current.
 2. Thegas sensor control apparatus as claimed in claim 1, wherein the H₂Oconcentration detection means detects the H₂O gas concentration from adifferential current obtained by subtracting the first pump current fromthe second pump current.
 3. A gas sensor control apparatus as claimed inclaim 1, wherein the current control means includes an analogcomputation circuit which performs analog computation for the feedbackcontrol on the basis of the electromotive force cell voltage; the analogcomputation circuit includes one or a plurality of circuit elementswhich determine the values of the group of control constants; and theconstant group setting means includes a switch which switches theconnection of the circuit elements of the analog computation circuit soas to set the group of control constants to either of the first group ofcontrol constants and the second group of control constants.
 4. The gassensor control apparatus as claimed in claim 1, wherein the currentcontrol means includes a computation section which performs digitalcomputation for the feedback control on the basis of the electromotiveforce cell voltage; and the constant group setting means sets the groupof control constants to either of the first group of control constantsand the second group of control constants.
 5. The gas sensor controlapparatus as claimed in claim 1, wherein the feedback control is PID(proportional-integral-derivative) control; and the group of controlconstants includes at least one of a proportionality constant, anintegration constant, and a differentiation constant for the PIDcontrol.